Oxygen to expand burner combustion capability

ABSTRACT

A burner at which fuel cannot be combusted with air as the only source of oxygen for combustion in a stable flame at the burner when the feed rate of the fuel is too low, when the fuel is fed at too high an air-to-fuel mass ratio, when the fuel contains too high an amount of inert matter, or when the specific energy content of the fuel is too low, is modified by supplying oxidant containing more than 21 vol. % oxygen into the base of a flame at the burner, whereupon such fuels can be combusted at the burner.

FIELD OF THE INVENTION

The present invention relates to combustion of carbonaceous fuel in acombustion device such as a coal-tired utility boiler.

BACKGROUND OF THE INVENTION

Combustion systems are usually designed to burn fuel that ischaracterized in that one or more properties of the fuel relevant tocombustion of the fuel lies within a range of values. The combustionsystem generally performs at its highest efficiency when the fuelcharacteristics fall within the ranges for which the system is designed.Attempts to combust fuel having one or more characteristics outside therange for that characteristic might encounter equipment limitations thatprevent the system from reaching design capacity or might lead tonegative side effects resulting in operational problems, higheremissions and increased maintenance. Features that may be affected byattempts to combust fuels having one or more characteristics outside ofthe ranges for which the system is designed include stability of thecombustion process, flame heat release pattern, combustion efficiency,NOx emissions, and air and flue gas fan capacity.

One example of a characteristic for which a combustion system is oftendesigned is the mass flow rate of fuel into the combustion chamber ofthe system. Most solid fuel fired combustion systems (for example,systems that combust coal in a boiler) use air-swept pulverizers to dryand pulverize the fuel, which is then carried into the combustionchamber and ignited. Since the transport medium for the pulverized fuelis air, the pulverizers and the fuel piping are designed to achieve atleast a minimum air velocity to avoid settling of fuel particles in theflowing stream of transport air. This design condition then requiresoperating the pulverizer with at least a minimum air flow rate, evenunder conditions of low fuel mass flow rates. Maintaining this minimumair flow rate thus dilutes the air/fuel mixture, under conditions of lowfuel mass flow rates, to a degree that stable combustion can not beattained and the burner flame gradually extinguishes. This is typicallythe case at loads below 30% of the full load capacity of the pulverizer.Practitioners have found it necessary to use auxiliary fuel (such asnatural gas or oil) to maintain flame and combustion stability when thefuel is being fed at such low rates.

A combustion method that permits combustion to be maintained with astable flame, even at fuel mass flow rates below those for which thesystem is designed, would thus be useful.

Other examples of characteristics for which a combustion system is oftendesigned are the content of inert (i.e. not combustible) matter (whethersolid, such as ash and minerals, or liquid, typically water), and thespecific energy value of the fuel, i.e. the amount of energy obtainableupon combustion of the combustible matter present per unit mass ofcombustible matter. Fuels that contain more inert matter than the rangeof inert matter for which the combustion system is designed, and fuelsthat have a specific energy value below the range of specific energyvalues for which the system is designed, when fed into the combustionsystem, cause many of the problems such as inability to maintaincombustion with a stable flame.

Fuels that may lead to such operational problems may nonetheless have aneconomical advantage over fuels that conform to the designspecifications of the system. Thus, a combustion method that permitscombustion to be maintained with a stable flame, even with fuel thatcontains too high a proportion of inert matter or too low a specificenergy value for the system, would thus be useful.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a method of modifying operationof a burner, comprising

-   -   (A) providing a burner through which a stream of air mixed with        particulate solid carbonaceous fuel, and one or more streams of        air other than the air mixed with said fuel, can be fed and        combusted in a stable flame at said burner, but        wherein maintaining said stable flame at said burner when air        provided in said streams is the only source of oxygen for said        combustion requires that the fuel satisfy one or more conditions        in that one or more of (1) the mass flow rate of the fuel        through said burner, (2) the fuel-to-air ratio of the stream of        air mixed with fuel that is fed through the burner, (3) the        content of combustible (Le. non-inert) matter in the fuel,        and (4) the specific energy value of the fuel, must be at least        sufficient for said stable flame to be maintained at said        burner,    -   (B) inserting through said burner a lance the outlet end of        which is positioned to eject gas into the base of a flame at        said burner,    -   (C) feeding through said burner a stream of air mixed with        particulate solid fuel which does not satisfy at least one of        said conditions and therefore cannot be combusted in a stable        flame at said burner in air as the only source of oxygen for        combustion, and feeding through said burner said one or more        streams of air other than the air mixed with said fuel,    -   (D) feeding gaseous oxidant comprising more than 21 vol. %        oxygen through the outlet end of said lance, and    -   (E) combusting said fuel and air fed in step (C) with said        oxidant fed in step (D) in a stable flame at said burner,        wherein said oxidant is fed into the base of said flame at a        mass flow rate that maintains said stable flame.

Another aspect of the present invention is a method of operating aburner, comprising

-   -   (A) providing a burner through which a stream of air mixed with        particulate solid carbonaceous fuel, and one or more streams of        air other than the air mixed with said fuel, can be fed and        combusted in a stable flame at said burner, but        wherein maintaining said stable flame at said burner when air        provided in said streams is the only source of oxygen for said        combustion requires that the fuel satisfy one or more conditions        in that one or more of (1) the mass flow rate of the fuel        through said burner, (2) the fuel-to-air ratio of the stream of        air mixed with fuel that is fed through the burner, (3) the        content of combustible matter in the fuel, and (4) the specific        energy value of the fuel, must be at least sufficient for said        stable flame to be maintained at said burner,    -   (B) feeding through said burner a stream of air mixed with        particulate solid fuel which does not satisfy at least one of        said conditions and therefore cannot be combusted in a stable        flame at said burner in air as the only source of oxygen for        combustion, and feeding through said burner said one or more        streams of air other than the air mixed with said fuel, and    -   (C) combusting said fuel and air fed in step (B) in a stable        flame at said burner while also feeding a stream of gaseous        oxidant comprising more than 21 vol. % oxygen into the base of        said flame at said burner, wherein said oxidant combusts with        said fuel and air, wherein said oxidant is fed into the base of        said flame at a mass flow rate that maintains said stable flame.

In both of the aforementioned aspects of the present invention, thestream of gaseous oxidant comprising more than 21 vol. % oxygen is inaddition to the fuel-air stream and the one or more streams of air otherthan the air mixed with the fuel in the fuel-air stream.

In both of the aforementioned aspects of the present invention, thecombustion that is made possible by feeding the gaseous oxidant isadvantageously carried out without feeding supplemental fuel in gaseous,liquid or solid form such as natural gas or methane, fuel oil or liquidhydrocarbons, or solids which contain a higher content of volatilizablematter than is contained in the particulate solid fuel that is fed andcombusted in the practice of this invention.

As used herein, that a flame is “stable” means that, once it isestablished under a given set of combustion conditions, it continues toburn indefinitely under those combustion conditions.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of one embodiment of apparatus withwhich the present invention can be practiced.

FIG. 2 is a cross-sectional view a burner useful in the embodiment ofFIG. 1.

FIG. 3 is a cross-sectional view of the burner of FIG. 2, showingmodification of that embodiment in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a representative combustion system with which the presentinvention can be practiced. The system includes combustion device 1,such as a coal-fired boiler. The fuel in this illustration is coal, butfuels with which the present invention is useful include any matter thathas heating value, i.e. that liberates heat upon combustion. The terms“fuel” and “fuel solids” used herein refer to the matter that is fed tobe combusted, including the combustible constituents thereof as well asany noncombustible constituents that are present.

Combustion device 1 houses combustion chamber 3, which is typically aspace that can withstand the high temperatures that are attained by thecombustion that is carried out in combustion chamber 3. The combustionchamber can be made of, or be lined with, refractory material, or it canbe contained by walls of tubes that carry material such as water thatabsorbs heat from the combustion chamber. Products of the combustionpass out of combustion chamber through flue 5. The heat that isgenerated by the combustion can be used in any of various ways (notshown in FIG. 1) such as forming steam in pipes that surround combustionchamber 3 or that are arrayed across flue 5.

Burner 11 is provided through a surface of combustion device 1. Inactual practice, anywhere from 1 to 20 or more burners may be provided,depending on the size of the installation. Furthermore, the burners canbe wall-mounted, roof-mounted, or corner-mounted. Fuel-air stream 12comprising a mixture of fuel and air, and air stream 13, are fed throughburner 11 and combusted in combustion chamber 3. The combustion formsflame 15 whose base is at the burner. Optional overtire air stream 14 ofair is fed into combustion chamber 3 downstream from flame 15, betweenflame 15 and flue 5. When more than one burner is employed, the airstreams 13 (and overtire air streams 14, when used) can be fed from acommon windbox or plenum (not shown) which is conventional in currentindustrial practice.

The fuel-air stream 12 can be formed in unit 17, which in manyembodiments is a pulverizer in which the fuel 18 is pulverized intoparticulate form that can be carried in a stream of transport air, andin which the fuel is mixed with air 19 which serves as transport air andwhich also provides some oxygen for combustion. The pulverizer typicallyhas a maximum mass flow rate of fuel (termed the “full load”) at whichit can produce fuel-air stream 12. Unit 17 can instead be apparatuswhich forms the fuel-air stream by combining a stream of alreadypulverized particulate fuel with a stream of transport air.

FIGS. 2 and 3 depict in cross-section one embodiment of a burner 11which is representative of burners with which the present invention canbe practiced. Passage 22 conveys the fuel-air stream 12 toward and intocombustion chamber 3, where it combusts in flame 15 as shown in FIGS. 1and 3. Passage or passages 23 convey air stream 13 toward and intocombustion chamber 3, where the air provides oxygen for combustion inflame 15. Optional passage or passages 24 convey secondary air towardand into combustion chamber 3, where the secondary air can participatein the combustion.

The burners depicted in FIGS. 2 and 3 are preferably circular, withpassage 22 disposed along the central axis of the burner. In oneembodiment there is one passage 23 which is annular and concentricallylocated completely around passage 22. In other embodiments, there can betwo or more passages 23, each terminating in its own opening intocombustion chamber 3. Likewise, the optional secondary air can beprovided through one passage 24 that is concentrically locatedcompletely around passage 22, or through two or more separate passageseach of which has its own opening into combustion chamber 3.

FIG. 3 depicts the burner of FIG. 2 which has been modified inaccordance with the present invention. Reference numerals that appear inboth FIGS. 2 and 3 have the same meanings for the embodiment of FIG. 3as for the embodiment of FIG. 2.

The embodiment of FIG. 3 includes lance 31 which is situated withinpassage 22. Lance 31 ends at opening 33 which is situated to feed gasout of opening 33 into the base 35 of flame 15. The other end of lance31 is connected to a supply of oxidant which is equipped with suitablevalves and controls so that it provides a stream of oxidant into lance31 when desired and at the flow rate desired. The oxidant should containmore than 21 vol. % oxygen, and preferably contains more than 30 vol. %oxygen and more preferably at least 90 vol. % oxygen. The oxidant can besupplied from a suitable storage tank, or can be provided by combining astream of air with a stream of commercially pure oxygen (e.g. 99 vol. %or higher purity oxygen) in amounts relative to each other thatestablish the desired oxygen content.

The present invention can be practiced in the following manner to modifya burner so that particulate solid carbonaceous fuel can be combusted atthe burner even though the fuel is fed at a fuel solids mass flow rateso low that combustion of the fuel at the burner with air as the onlysource of oxygen for combustion cannot be maintained in a stable flameat the burner.

The minimum fuel solids mass flow rate is determined, for that burner,at which combustion of the fuel with air as the sole source of oxygenfor combustion could be maintained in a stable flame at the burner. Oneway to determine this rate is to determine, at the minimum airflow ratethat is necessary for operation of the burner, the minimum content offuel solids in that airflow at which combustion of the fuel fed in thatairflow can be maintained in a stable flame at the burner, with air asthe only source of oxygen for combustion. The combination of the minimumairflow rate and the minimum fuel solids content establishes a minimumfuel solids mass flow rate at which combustion in air could bemaintained at the burner in a stable flame.

Lance 31 or equivalent conduit is placed through burner 22 as shown inFIG. 3, with its outlet end positioned at the opening of the burner atits other end connected (through conventional valves and controlsenabling control of the flow rate and enabling one to turn the flow onand off) to a source of oxidant containing more than 21 vol. % oxygen,conveniently at least 30 vol. % oxygen, and preferably at least 90 vol.% oxygen.

Then, a fuel-air stream is fed through the burner at a solids mass flowrate which is lower than that minimum established as described above,combustion air is fed through the burner (for instance, through passageor passages 23 of the burner in FIG. 3), and oxidant containing morethan 21 vol. % oxygen, conveniently at least 30 vol. % oxygen, andpreferably at least 90 vol. % oxygen, is fed through lance 31.. Ifcombustion at the burner has already been established, the oxidantemerging at outlet 33 is fed into the base 35 of the flame at theburner. If combustion at the burner has not already been established,the mixture of the fuel-air stream, the combustion air stream, and theoxidant is ignited, and the oxidant emerging at outlet 33 is fed intothe base 35 of the flame at the burner.

The mass flow rate at which oxygen is fed into the base of the flame isadjusted to determine a value at which combustion of the fuel ismaintained in a stable flame at the burner. Then, the flow rate ofoxygen is held at that level, or is increased to ensure stablecombustion even in the event of fluctuations of the mass flow rate ofthe fuel. Typically, the amount of oxygen that is present in the oxidantemerging from outlet 33 into the base 35 of the flame is 1% to 25% ofthe total stoichiometric amount required to completely combust thecombustible portion of the fuel that is fed. If desired, the oxygencontent of the oxidant can be adjusted to accommodate the needs of thesituation; as the feed rate decreases, increasing the oxygen content ofthe oxidant will generally be needed to maintain stable combustion ofthe fuel.

This embodiment of the invention is expected to permit combustion in astable flame at the burner to be maintained even when the fuel solidsmass flow rate corresponds to 30% or less of the minimum fuel solidsmass flow rate needed for stable combustion to be maintained when air isused as the only source of oxygen for combustion. The minimum fuelsolids mass flow rate at which this invention becomes applicable,whether expressed as an absolute figure or as a percentage of themaximum flow rate, varies from one unit to another but can readily bedetermined experimentally for any unit.

The present invention can be practiced in the following manner to modifya burner so that particulate solid carbonaceous fuel can be combusted inthe burner even though the stream of air mixed with fuel which is fedthrough the burner (such as from a pulverizer) has an air-to-fuel massratio so high, such as 2.5 or higher or even 3.0 or higher (i.e. thatmight be encountered upon “turndown” of the combustion rate), thatcombustion of the fuel in the burner with air as the only source ofoxygen for combustion cannot be maintained in a stable flame at theburner. (it will of course be recognized that references herein to anair-to-fuel ratio too high to enable a stable flame, and to afuel-to-air ratio needing to be above a value to enable a stable flame,are simply different ways of expressing the same point.)

The maximum air-to-fuel mass ratio in the stream of air mixed with fuelthat is fed through the burner is determined, for that burner, at whichcombustion of the fuel can be maintained in a stable flame at theburner, with air as the only source of oxygen for combustion.

Lance 31 or equivalent conduit is placed through burner 22 as shown inFIG. 3, with its outlet end positioned at the opening of the burner atits other end connected (through conventional valves and controlsenabling control of the flow rate and enabling one to turn the flow onand off) to a source of oxidant containing more than 21 vol. % oxygen,conveniently at least 30 vol. % oxygen, and preferably at least 90 vol.% oxygen. Then, a fuel-air stream is fed through the burner wherein theair-to-fuel ratio of that stream is higher than that maximum establishedas described above, combustion air is fed through the burner (forinstance, through passage or passages 23 of the burner in FIG. 3), andoxidant containing more than 21 vol. % oxygen, conveniently at least 30vol. % oxygen, and preferably at least 90 vol. % oxygen, is fed throughlance 31. If combustion at the burner has already been established, theoxidant emerging at outlet 33 is fed into the base 35 of the flame atthe burner. If combustion at the burner has not already beenestablished, the mixture of the fuel-air stream, the combustion airstream, and the oxidant is ignited, and the oxidant emerging at outlet33 is fed into the base 35 of the flame at the burner.

The mass flow rate at which oxygen is fed into the base of the flame isadjusted to determine a value at which stable combustion of the fuel ismaintained in a flame at the burner. Then, the flow rate of oxygen isheld at that level, or increased to ensure stable combustion even in theevent of fluctuations of the content of noncombustible matter in thefuel. Typically, the amount of oxygen that is present in the oxidantemerging from outlet 33 into the base 35 of the flame is 1% to 25% ofthe total stoichiometric amount required to completely combust thecombustible portion of the fuel fed. If desired, the oxygen content ofthe oxidant can be adjusted to accommodate the needs of the situation;as the air-to-fuel ratio of the fuel feed stream increases, increasingthe oxygen content of the oxidant will generally be needed to maintainstable combustion of the fuel.

The maximum air-to-fuel ratio in the fuel feed stream, above which thepresent invention becomes applicable, varies from one unit to anotherbut can readily be determined experimentally for any given unit. Ingeneral, combustion of fuel fed in streams of air mixed with the fuelwherein the air-to-fuel ratio is below about 2.0 is less likely to needthe assistance provided by the present invention, whereas the ability ofthe present invention to achieve combustion of fuel fed in feed streamshaving higher air-to-fuel ratios is likely to be realized with fuel feedstreams fed at air-to-fuel ratios of 2.5 or higher, and even more likelywhen fed at air-to-fuel ratios of 3.0 or higher.

The present invention can be practiced in the following manner to modifya burner so that particulate solid carbonaceous fuel can be combusted inthe burner even though the fuel contains an amount of noncombustible(inert) material so high, up to 70 or 75 wt. %, or even 80 to 90 wt. %,that combustion of the fuel in the burner with air as the only source ofoxygen for combustion cannot be maintained in a stable flame at theburner. Fuel containing that much inert material can be found or formednaturally, or can be formed by blending fuel with lesser (or no) inertmaterial with inert material or with fuel containing even higher amountsof inert material.

The maximum content of noncombustible matter in the fuel is determined,for that burner, at which combustion of the fuel can be maintained in astable flame at the burner, with air as the only source of oxygen forcombustion.

Lance 31 or equivalent conduit is placed through burner 22 as shown inFIG. 3, with its outlet end positioned at the opening of the burner atits other end connected (through conventional valves and controlsenabling control of the flow rate and enabling one to turn the flow onand off) to a source of oxidant containing more than 21 vol. % oxygen,conveniently at least 30 vol. % oxygen, and preferably at least 90 vol.% oxygen. Then, a fuel-air stream is fed through the burner wherein thecontent of noncombustible matter in the fuel is higher than that maximumestablished as described above, combustion air is fed through the burner(for instance, through passage or passages 23 of the burner in FIG. 3),and oxidant containing more than 21 vol. % oxygen, conveniently at least30 vol. % oxygen, and preferably at least 90 vol. % oxygen, is fedthrough lance 31. If combustion at the burner has already beenestablished, the oxidant emerging at outlet 33 is fed into the base 35of the flame at the burner. If combustion at the burner has not alreadybeen established, the mixture of the fuel-air stream, the combustion airstream, and the oxidant is ignited, and the oxidant emerging at outlet33 is fed into the base 35 of the flame at the burner.

The mass flow rate at which oxygen is fed into the base of the flame isadjusted to determine a value at which stable combustion of the fuel ismaintained in a flame at the burner. Then, the flow rate of oxygen isheld at that level, or increased to ensure stable combustion even in theevent of fluctuations of the content of noncombustible matter in thefuel. Typically, the amount of oxygen that is present in the oxidantemerging from outlet 33 into the base 35 of the flame is 1% to 25% ofthe total stoichiometric amount required to completely combust thecombustible portion of the fuel fed. If desired, the oxygen content ofthe oxidant can be adjusted to accommodate the needs of the situation;as the percentage of combustible matter in the fuel decreases,increasing the oxygen content of the oxidant will generally be needed tomaintain stable combustion of the fuel.

The maximum noncombustible matter content above which the presentinvention becomes applicable varies from one unit to another but canreadily be determined experimentally for any given unit. In general,combustion of fuels having noncombustible matter content below about 30wt. % is less likely to need the assistance provided by the presentinvention, whereas the ability of the present invention to achievecombustion of fuel having high noncombustible matter content is likelyto be realized with fuel containing 35 wt. % or higher noncombustiblematter, and even more likely with fuel containing 40 wt. % or highernoncombustible matter.

The present invention can be practiced in the following manner to modifya burner so that particulate solid carbonaceous fuel can be combusted inthe burner even though the specific energy content of the fuel (e.g. BTUper pound of fuel) is so low that combustion of the fuel in the burnerwith air as the only source of oxygen for combustion cannot bemaintained in a stable flame at the burner.

The minimum specific energy content of the fuel is determined at whichcombustion of the fuel can be maintained in a stable flame at theburner, with air as the only source of oxygen for combustion.

Lance 31 or equivalent conduit is placed through burner 22 as shown inFIG. 3, with its outlet end positioned at the opening of the burner atits other end connected (through conventional valves and controlsenabling control of the flow rate and enabling one to turn the flow onand off) to a source of oxidant containing more than 21 vol. % oxygen,conveniently at least 30 vol. % oxygen, and preferably at least 90 vol.% oxygen.

Then, a fuel-air stream is fed through the burner wherein the specificenergy content of the fuel is lower than that minimum established asdescribed above, combustion air is fed through the burner (for instance,through passage or passages 23 of the burner in FIG. 3), and oxidantcontaining more than 21 vol. % oxygen, conveniently at least 30 vol. %oxygen, and preferably at least 90 vol. % oxygen, is fed through lance31. If combustion at the burner has already been established, theoxidant emerging at outlet 33 is fed into the base 35 of the flame atthe burner. If combustion at the burner has not already beenestablished, the mixture of the fuel-air stream, the combustion airstream, and the oxidant is ignited, and the oxidant emerging at outlet33 is fed into the base 35 of the flame at the burner.

The mass flow rate at which oxygen is fed into the base of the flame isadjusted to determine a value at which stable combustion of the fuel ismaintained in a flame at the burner. Then, the flow rate of oxygen isheld at that level, or increased to ensure stable combustion even in theevent of fluctuations of the specific energy content of the fuel.Typically, the amount of oxygen that is present in the oxidant emergingfrom outlet 33 into the base 35 of the flame is 1% to 25% of the totalstoichiometric amount required to completely combust the combustibleportion of the fuel fed. If desired, the oxygen content of the oxidantcan be adjusted to accommodate the needs of the situation; as thespecific energy content of the fuel decreases, increasing the oxygencontent of the oxidant will generally be needed to maintain stablecombustion of the fuel.

The minimum specific energy content below which the present inventionbecomes applicable varies from one unit to another but can readily bedetermined experimentally for any given unit. In general, combustion offuels having specific energy content above about 10,000 BTU/pound isless likely to need the assistance provided by the present invention,whereas the ability of the present invention to achieve combustion offuel having low specific energy content is likely to be realized withfuel having a specific energy content of 8,000 BTU/pound or lower, asdetermined from a dried fuel sample, and even more likely with fuelhaving a specific energy content of 6,000 BTU/pound or lower asdetermined from a dried fuel sample.

What is claimed is:
 1. A method of modifying operation of a burner,comprising (A) providing a burner through which a stream of air mixedwith particulate solid carbonaceous fuel, and one or more streams of airother than the air mixed with said fuel, can be fed and combusted in astable flame at said burner, but wherein maintaining said stable flameat said burner when air provided in said streams is the only source ofoxygen for said combustion requires that the fuel satisfy one or moreconditions in that one or more of (1) the mass flow rate of the fuelthrough said burner, (2) the fuel-to-air ratio of the stream of airmixed with fuel that is fed through the burner, (3) the content ofcombustible matter in the fuel, and (4) the specific energy value of thefuel, must be at least sufficient for said stable flame to be maintainedat said burner, (B) inserting through said burner a lance the outlet endof which is positioned to eject gas into the base of a flame at saidburner, (C) feeding through said burner a stream of air mixed withparticulate solid fuel which does not satisfy at least one of saidconditions and therefore cannot be combusted in a stable flame at saidburner in air as the only source of oxygen for combustion, and feedingthrough said burner said one or more streams of air other than the airmixed with said fuel, (D) feeding gaseous oxidant comprising more than21 vol. % oxygen through the outlet end of said lance, and (E)combusting said fuel and air fed in step (C) with said oxidant fed instep (D) in a stable flame at said burner, wherein said oxidant is fedinto the base of said flame at a mass flow rate that maintains saidstable flame.
 2. A method according to claim 1 wherein the gaseousoxidant fed in step (D) comprises at least 90 vol. % oxygen.
 3. A methodof modifying operation of a burner, comprising (A) providing a burnerthrough which a stream of air mixed with particulate solid carbonaceousfuel, and one or more streams of air other than the air mixed with saidfuel, can be fed and combusted in a stable flame at said burner, (B)determining the maximum content of noncombustible matter of particulatesolid carbonaceous fuel above which combustion of said fuel with air asthe only source of oxygen for said combustion cannot be maintained in astable flame at said burner, (C) inserting through said burner a lancethe outlet end of which is positioned to eject gas into the base of aflame at said burner and the inlet end of which is connected to a sourceof gaseous oxidant comprising more than 21 vol. % oxygen, (D) feedingthrough said burner a stream of air mixed with said particulate solidfuel having a content of noncombustible matter above said maximum, andfeeding through said burner said one or more streams of air other thanthe air mixed with said fuel, (E) feeding gaseous oxidant comprisingmore than 21 vol. % oxygen through the outlet end of said lance, and (F)combusting said fuel and air fed in step (D) with said oxidant fed instep (E) in a stable flame at said burner, wherein said oxidant is fedinto the base of said flame at a mass flow rate that maintains saidstable flame.
 4. A method according to claim 3 wherein the gaseousoxidant fed in step (D) comprises at least 90 vol. % oxygen.
 5. A methodof operating a burner, comprising (A) providing a burner through which astream of air mixed with particulate solid carbonaceous fuel, and one ormore streams of air other than the air mixed with said fuel, can be fedand combusted in a stable flame at said burner, (B) feeding through saidburner a stream of air mixed with said particulate solid fuel having acontent of noncombustible matter above the maximum content ofnoncombustible matter of particulate solid carbonaceous fuel above whichcombustion of said fuel with air as the only source of oxygen for saidcombustion cannot be maintained in a stable flame at said burner, andfeeding through said burner said one or more streams of air other thanthe air mixed with said fuel, (C) combusting said fuel and air fed instep (B) in a stable flame at said burner while feeding gaseous oxidantcomprising more than 21 vol. % oxygen into the base of said flame atsaid burner, wherein said oxidant combusts with said fuel and air,wherein said oxidant is fed into the base of said flame at a mass flowrate that maintains said stable flame.
 6. A method according to claim 5wherein the gaseous oxidant fed in step (C) comprises at least 90 vol. %oxygen.